This paper presents a clinical evaluation of a newly-developed colonoscopy training simulator that includes a specialized haptic interface to transfer force-feedback through a long and flexible colonoscope tube. The colonoscopy simulator has a 2-DOF haptic device with folding guides, which can transfer large decoupled forces of the colonoscopy simulation to the user. Centerline-based parametric colon models are developed to compute collision detection and response in real-time. The clinical study included 22 subjects consisting of colonoscopy experts (n=3), fellows (n=4), residents (n=6), and medical school students (n=9). The study shows that the developed colonoscopy simulator can effectively improve the skills of the residents and medical school students, although no statistically significant training-effects can be established for the fellows.